In general, a decoding method for obtaining original signals of a plurality of channels by decoding an input signal obtained by converting the original signals of the plurality of channels into a down-mix main signal, a residual signal, and spatial information and encoding the main signal, the residual signal, and the spatial information has been used.
For example, as a method for encoding surround audio signals of 5.1 ch, an MPEG surround method standardized by ISO/IEC (ISO/IEC 23003-1) has been used. In the MPEG surround method, a surround audio signal is converted into a single-channel or two-channel down-mix signal (main signal), a residual signal, and spatial information and the main signal, the residual signal, and the spatial information are encoded. An MPEG surround decoder obtains surround audio signals by decoding the down-mix signal, the residual signal, and the spatial information.
In such a general decoding method using a main signal, a residual signal, and spatial information, when an input signal is decoded, the residual signal is used in a frequency band determined in advance at a time of encoding and a decorrelation signal generated from the decoded main signal is used instead of the residual signal in other frequency bands.